By adding different content of B to AlMg5Si2Mn alloy, the influence on the microstructure, tensile properties and hardness of the alloy was studied. The results show that the eutectic Mg2 Si in AlMg5Si2Mn alloy have obvious metamorphism by adding proper amount of B. When 0.1%B is added, the primary alpha-al is obviously refined and partially transformed into equiaxial shape. The size and spacing of the eutectic Mg2 Si in long needles and large sheets decrease obviously. The tensile strength of 235 MPa is increased to 310 MPa, which is increased by 32%. When the content of B reached 0.22%, eutectic Mg2 Si is all transformed into point or fiber. At this time, elongation increased from 3.9% without modification to 9.1% and 130.9%, and PSE increase from 925.9 MPa·% without B to 2 684.5 MPa·%.